Disclosed in Australian Innovation Patent No. 2007100714 is a variable aperture screen. The screen has a base with a plurality of magnetic bars that are spaced by apertures. Steel items, in the material passing through the screen, are removed by being attracted to the bars. The bars consist of two sets, with relative movement between the sets adjusting the apertures between the bars. Each set of bars is mounted on a base so that the bars of each set are moved in unison.
A disadvantage of the above described screen is that it is generally complex and therefore expensive to manufacture. A further disadvantage is that frequently a set of bars that is being adjusted may be difficult to move.
Many situations exist in the Food industry where, because of increased regulation and standards, it becomes necessary to upgrade existing older style grate type magnets. A number of problems exist in respect of carrying out the upgrade, those problems include:                1. Existing old type grate magnets are often in a housing and removable as a ‘drawer’ for external cleaning. Where access is not ideal, this can be difficult even dangerous.        2. The grate bars may be covered by non magnetic “sleeves” and the magnet elements withdrawn substantially out of the sleeves until the extracted magnetics drop off Sleeves reduce available magnetic strength.        3. The magnetic grate bars may be pulled through external cleaning devices until collected magnetics material drop off. If cleaning devices are tight and efficient, pulling a number of bars simultaneously through cleaning devices is difficult unless power assisted.        
Due to varying product characteristics and lump sizes it is difficult for manufacturers of grate magnets to initially propose an ideal gap between magnet bars which both minimizes blockage and maximizes magnetic separation efficiency. Prior devices provide long horizontal slot/s or numbers of holes into which bars can be added to adjust the gap or removed to increase the gap between the bars. Uncovered holes or portion of slots is also not suitable for hygienic applications. Because modern magnet bars are extremely powerful and forcefully attracted to each other, it is unsafe and impractical to completely remove or unscrew such bars in order to effect gap adjustment.
In a column of falling material, there may be different densities of material. The centre of the column may be denser than the extremities.
Issues calling for replacement of grate magnets include:                Lack of magnetic strength as in the case of old style or “sleeve type magnets”        Difficulty of cleaning where magnet bars ends are bounded by or attached to a “frame”.        Difficulty in removing, replacing and cleaning a “drawer” type grate magnet.        Lack of magnetic coverage of the product stream because bars are too widely spaced.        Product blocking the grate due to the magnetic bars being too close to each other.        Unequal abrasive wear on the outer tube containing the magnets.        The existing grate not being sufficiently hygienic to satisfy today's food grade standard.        
In the following description a screen is also intended to mean a grate.
Magnetic separation devices are known in which a substance (such as a gas, liquid or powder) is passed through the device for the purposes of removing ferrous and other magnetic materials. These previous devices have a number of magnetic fingers that extend transverse of the material flowing through the device.
A first disadvantage of known liquid or powder magnetic separation devices is that they are difficult to clean. A further disadvantage is that it is often necessary to support the magnetic bars at both ends, that is the bars are attached to an end plate as well to a door or lid. A still further disadvantage is that in the use of high powered magnets, when they are being placed in a machined outer housing of the device, the magnets are attracted and hold tightly to the sides of the device. This causes difficulties in respect of sealing and centrally placing the lid. The effect is a problem when the housing is a machined casting and becomes work hardened and therefore magnetically susceptible even though it is an austenitic stainless steel.